The present invention relates to a signal transmission method and a signal transmission device that allow one pair of signal lines to carry out bidirectional communication between a regulated power supply unit (ballast) that stably operates and controls a high intensity discharge (HID) lamp used in a video projection device such as a projector, and a setting unit that gives various kinds of commands to this regulated power supply unit, and relates also to a liquid crystal projector employing the method or device.
A color liquid crystal projector includes a high intensity discharge lamp (hereinafter, referred to as an HID lamp) such as a metal halide lamp and a regulated power supply unit that stably operates and controls the HID lamp. The liquid crystal projector includes also a color liquid crystal panel, a diaphragm (aperture), a converging lens, a screen, and so on that are arranged in that order in front of the HID lamp. Furthermore, the projector includes a power converter that converts a DC input into power necessary for the operation of the HID lamp, a high voltage generator that ignites the HID lamp, a controller that controls the operation of the HID lamp, and so on.
Light emitted from the HID lamp in response to the lighting of the HID lamp enters the color liquid crystal panel, and an image formed of the three primary colors of the color liquid crystal panel is projected via the diaphragm and converging lens onto the screen, which allows a color image to be displayed on the screen (refer to Japanese Patent Laid-open No. Hei 10-188896).
In addition, to the regulated power supply unit, various kinds of commands are transmitted from the setting unit coupled via signal lines to the regulated power supply unit. Moreover, response signals and so on are sent back from the regulated power supply unit to the setting unit.
A description will be made below on an existing signal transmission system between a regulated power supply unit and a setting unit with reference to FIG. 13.
In FIG. 13, a regulated power supply unit 81 that stably operates and controls an HID lamp, and a setting unit 82 that gives various kinds of commands to the regulated power supply unit 81 are shown.
The regulated power supply unit 81 includes a microcomputer 811, a transmission photo-coupler 812 and a reception photo-coupler 813. The microcomputer 811 has a function to control and manage the entire regulated power supply unit 81 and a function to control communication with the setting unit 82. The photo-couplers 812 and 813 are used for the communication with the setting unit 82.
The setting unit 82 includes a microcomputer 821 that has a function to control and manage the entire setting unit 82 and a function to control communication with the regulated power supply unit 81.
The anode of an input light emitting diode 812a included in the transmission photo-coupler 812 is coupled via a resistor R1 to a transmission terminal Tx of the microcomputer 811, while the cathode of the light emitting diode 812a is connected to a ground terminal GND.
The collector and emitter of an output phototransistor 812b included in the transmission photo-coupler 812 are separately connected to one end of a pair of transmission-only signal lines L1 and L2, respectively, that interconnect the regulated power supply unit 81 and the setting unit 82. The other end of the signal line L1 is connected to a +5 V power supply on the setting unit side. The other end of the signal line L2 is connected to a reception terminal Rx of the microcomputer 821 of the setting unit 82, and is coupled via a resistor R5 to the ground terminal GND.
The collector of an output phototransistor 813b included in the reception photo-coupler 813 is connected to a reception terminal Rx of the microcomputer 811, and is coupled via a resistor R2 to a +5 V power supply. The emitter of the phototransistor 813b is connected to the ground terminal GND.
The anode of an input light emitting diode 813a included in the reception photo-coupler 813 is coupled via a resistor R3 to one end of one signal line L3 of a pair of reception-only signal lines L3 and L4 that interconnect the regulated power supply unit 81 and the setting unit 82. The cathode of the light emitting diode 813a is connected to one end of the other signal line L4. The other end of the signal line L3 is connected to the +5 V power supply on the setting unit side. The other end of the signal line L4 is coupled via a resistor R4 to a transmission terminal Tx of the microcomputer 821 of the setting unit 82.
In the signal transmission circuit shown in FIG. 13, when the regulated power supply unit 81 lights the HID lamp, the potential of the transmission terminal Tx of the microcomputer 821 in the setting unit 82 is switched to the “Low” level. Upon this switching, a current flows from the +5 V power supply through the light emitting diode 813a in the reception photo-coupler 813 via the signal lines L3 and L4. Thus, the light emitting diode 813a implements light emission operation, and simultaneously the phototransistor 813b that has received the emitted light is turned on. This ON signal is captured in the microcomputer 811 via its reception terminal Rx. The microcomputer 811 determines the status of the ON signal and controls the regulated power supply unit 81 so that the HID lamp is lit.
If the HID lamp is lit, an ignition detection circuit in the regulated power supply unit 81 detects the state where the HID lamp has been ignited due to a high voltage generated from a high voltage generator in the regulated power supply unit 81. Furthermore, based on the detection signal, the potential of the transmission terminal Tx of the microcomputer 811 in the regulated power supply unit 81 is switched to the “High” level. Upon this switching, the light emitting diode 812a in the transmission photo-coupler 812 implements light emission operation, and simultaneously the phototransistor 812b that has received the emitted light is turned on. Thus, a current flows from the +5 V power supply via the signal lines L1 and L2 and is captured in the microcomputer 821 of the setting unit 82 via its reception terminal Rx. The microcomputer 821 determines whether or not the current is a lighting acknowledgement signal from the regulated power supply unit 81, and thereby can confirm whether or not the HID lamp has been lit.
In contrast, when the HID lamp is turned off, the potential of the transmission terminal Tx of the microcomputer 821 in the setting unit 82 is switched from the “Low” level to the “High” level. Due to this switching, the HID lamp is turned off.
In the above-described existing signal transmission system, however, two signal lines need to be prepared for each of the transmission system and reception system as shown in FIG. 13. In addition, if the signal transmission system is further provided with, besides the above-described functions to light and turn-off the HID lamp, a function to switch the power mode for the HID lamp between e.g. 100 W and 120 W and other functions, there is a need to separately prepare dedicated signal lines and photo-couplers for transmitting an instruction signal for the power mode switching and so on, for each kind of instruction signal. Therefore, the numbers of signal lines and photo-couplers for interconnecting the regulated power supply unit 81 and the setting unit 82 are significantly increased, and the number of components of the signal transmission circuit is also increased. Accordingly, the existing system involves a problem of being disadvantageous in terms of costs.